Anyone figure this out yet? I'm normally at home or at school/work, all of which has WiFi. Which way saves more power? Is the GSM service off when I'm on UMA (I assume not, because transitioning from UMA to GSM wouldn't be seamless if it weren't)? Is there any point to keep my WiFi on all the time if I'm not a heavy data user?

One of the selling features of the Tmobile router is it increases battery life versus regular routers. Unless your school has similarly set up routers, GSM is probably better for battery life. But if the GSM signal is not so strong, that might be worse.

Best to just try the two different setups and see what works best for you.